Photochromism of Diarylethene with Benzoxazole
NMR and MS data of 2-arylbenzoxazoles 5 with that of
a known model compounds finds data of both 1H NMR
and MS of 5 are in agreement with that found in the
literature.15 It indicates that 2-arylbenzoxazoles 5 can be
produced directly from phenolic Schiff bases derived (4)
in good yields (63-78%) in the conditions of base (NaOH)
and phototrigger (Hg high-pressure lamp, 500 W) in
acetonitrile. It is known16-19 that benzoxazole ring moi-
eties are often found in compounds that exhibit biological
activities, including antitumor, antimicrobial, and anti-
viral propterties. There are two general methods for
synthesizing 2-substituted benzoxazoles. One is the
coupling of 2-aminophenols with carboxylic acid deriva-
tives, which either is catalyzed by strong acids20 or
requires microwave conditions.21 The other is the oxida-
tive cyclization of phenolic Schiff bases derived from the
condensation of 2-aminophenols and aldehydes. In the
F IGURE 5. Photochromic bleaching of the band at 564 nm
with 560 nm light.
latter reactions, various oxidants such as DDQ,22 Mn-
SCHEME 2. P r ep a r a tion of 2-Ar ylben zoxa zoles
fr om Der ived P h en olic Sch iff Ba ses
- 25
(Oac)3,23 PhI(Oac)2,24 Th+ClO4
,
and O2 with activated
carbon (Darco KB)15 are used. The preparation of 2-aryl-
benzozaxoles from phenolic Schiff bases derived in the
condition of base and phototrigger has, as far as we know,
not been reported before. The investigation of photo-
physical property of 2-benzyl benzoxazole shows that the
absorption band of 2-benzyl benzoxazole (λmax ) 304 nm,
in acetonitrile) is blue-shifted as much as 46 nm (∆λmax
) 350 - 304 nm) by comparison with phenolic Schiff base
(λmax ) 350 nm, in acetonitrile). This result is similar to
that above, in which as much as 40 nm (∆λmax ) 362 -
322 nm) blue-shift is observed when 1a is converted to
3a in acetonitrile.
the band at 412 nm does not increase when the band at
564 nm decreases and only the band at 322 nm increases
in that case. All results indicate that the band at 322
nm can be attributed to a new chemical species that is
from 2a by irradiation with 410 nm light.
To explore the conversion of phenolic Schiff bases into
2-arylbenzoxazoles, the following tests are carried out.
First, the solution of 1a without adding NaOH base is
tested by irradiation with 313 nm light. It is found that
only the band of ring-closed form (1b) is detected as
shown in Figure 1. It indicates that 1a cannot be
converted to 3a in this case. Second, when a solution of
2a (obtained from 1a by addition of 1 equiv of NaOH
solution till the absorption intensity does not change) is
kept in darkness at room temperature for 24 h, no
remarkable change is detected by UV absorption, indicat-
ing 2a cannot be converted to 3a without phototrigger.
Third, irradiating the solution of 2a with 313 nm light
in the presence and absence of O2, respectively, gives
results that are the same. Last, different solvents (di-
choromethan, toluene) and different substrates are em-
ployed and similar results are obtained. All indicate that
both base and phototrigger play a key role in preparation
Further investigation confirms that the band at 564
nm has close relations with the band at 322 nm. When
the solution is irradiated with 313 nm light, the band at
564 nm appears and increases with decrease of the band
at 322 nm. Conversely the band at 322 nm increases with
decrease of band at 564 nm when above solution is
bleached with 560 nm light. What is more, no significant
photodegradation is detected when coloration and bleach-
ing of the solution as above is repeated for 10 times. The
photostationary equilibrium is achieved very quickly (0.5
min for the above solution) and the conversion is ca. 20%.
Next we turn our attention to figure out the molecular
structure of the new chemical species produced from
irradiation of 2a with 410 nm light. 1H NMR data shows
that the protons (OH, CHdN) of 1a appearing at 8.62
and 7.15 ppm, respectively, in the 1H NMR spectrum
disappeared in the 1H NMR spectra of new chemical
species. Moreover, the protons of new chemical species
in the thiophene ring and benzene ring shift downfield
as much as 0.72 and 0.69 ppm, respectively, by compari-
son with that of 1a . In addition, the mass spectroscopy
shows the relative abundance of molecular ion (m/z )
512) of the new chemical species is 100%. All indicate
that the new chemical species produced from 2a is
diarylethene with benzoxazole 3a , whose structure is
shown in Scheme 1.
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Lett. 2003, 5, 3713.
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(17) Sato, Y.; Yamada, M.; Yoshida, S.; Soneda, T.; Ishikawa, M.;
Nizato, T.; Suzuki, K.; Konno, F. J . Med. Chem. 1998, 41, 3015.
(18) Temiz, O.; Oren, I.; Sener, E.; Yalcin, I.; Ucarturk, N. Farmaco
1998, 53, 337.
(19) Sato, S.; Kajiura, T.; Noguchi, M.; Takehana, K.; Kobayashi,
T.; Tsuji, T. J . Antibiot. 2001, 54, 102.
(20) Terashima, M.; Ishii, M.; Kanalka, Y. Synthesis 1982, 484.
(21) Pottorf, R. S.; Chadha, N. K.; Katkevics, M.; Ozola, V.; Suna,
E.; Ghane, H.; Regberg, T.; Player, M. R. Tetrahedron Lett. 2003, 44,
175.
(22) Chang, J .; Zhao, K.; Pan, S. Tetrahedron Lett. 2002, 43, 951.
(23) Varma, R. S.; Kumar, D. J . Heterocycl. Chem. 1998, 35, 1539.
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To confirm the molecular structure of 3a and to test
the general preparation method above, phenolic Schiff
bases with different substituted group (Scheme 2) are
1
prepared and employed as template. Comparing the H
J . Org. Chem, Vol. 69, No. 15, 2004 5039